A mixture of 4,4′-dihydroxydiphenylsulfone and 2,4′-dihydroxydiphenylsulfone can be obtained by condensation with dehydration of phenol and sulfuric acid. 4,4′-Dihydroxydiphenylsulfone is known as the raw material for polyether sulfone which is an engineering plastic exhibiting excellent heat resistance and a monomer for improving heat resistance of polycarbonates. 2,4′-Dihydroxydiphenylsulfone is a useful compound which is used as a developer for heat-sensitive recording materials and, in particular, provides excellent heat-sensitive recording paper exhibiting excellent coloring property, causing little fog on the background and stored with stability.
The product of condensation with dehydration of phenol and sulfuric acid is a mixture of 4,4′-dihydroxydiphenylsulfone and 2,4′-dihydroxydiphenylsulfone. Therefore, it is necessary for obtaining highly pure 4,4′-dihydroxydiphenylsulfone or 2,4′-dihydroxydiphenylsulfone that these compounds are separated from each other. Various processes for the separation have been examined.
As for the process for separating 4,4′-dihydroxydiphenylsulfone, for example, a process in which a mixture of the isomers of dihydroxydiphenylsulfone is dissolved into phenol by heating, 4,4′-dihydroxydiphenylsulfone alone is separated by crystallization as an addition compound with phenol by cooling, and then crystals of 4,4′-dihydroxydiphenylsulfone are obtained by a treatment of heating, is proposed in Japanese Patent Application Laid-Open No. Showa 57(1982)-77667 as the process for separating a highly pure 4,4′-dihydroxydiphenylsulfone from a mixture of the isomers. In Japanese Patent Application Laid-Open No. Showa 50(1975)-106936, a process in which a product of the reaction between phenol and sulfuric acid is brought into contact with an aqueous solution of phenol having a concentration of 3 to 35% by weight, and 4,4′-dihydroxy-diphenylsulfone is separated by crystallization, is proposed as the process for producing 4,4′-dihydroxydiphenylsulfone having a small content of the isomer. However, in accordance with these processes, it is difficult that 2,4′-dihydroxydiphenylsulfone is separated from the residual mixture obtained after the separation of 4,4′-dihydroxydiphenylsulfone.
In Japanese Patent Application Laid-Open No. Heisei 10(1998)-25277, a process in which phenol and a sulfonating agent are brought into reaction in a solvent of o-dichlorobenzene, the amount of the unreacted phenol in the reaction mixture is adjusted to 2 to 20% by weight based on the total of the amounts of the unreacted phenol and o-dichlorobenzene, and the total of the amounts of the unreacted phenol and o-dichlorobenzene is adjusted to 2 to 7 times the theoretical yield of dihydroxydiphenylsulfones when the reaction is completed so that 4,4′-dihydroxydiphenylsulfone is separated by crystallization from the reaction mixture, and 2,4′-dihydroxydiphenylsulfone is isolated form the filtrate, is proposed as the process for producing a combination of highly pure 2,4′-dihydroxydiphenylsulfone and 4,4′-dihydroxydiphenylsulfone industrially advantageously. However, since this process uses the chlorine-based organic solvent, this process requires a facility for recovery of the solvent and, moreover, tends to cause problems on the environment.
In Japanese Patent Application Laid-Open No. Heisei 9(1997)-40635, a process in which, in an aqueous solution of a mixture of 2,4′-dihydroxydiphenylsulfone and 4,4′-dihydroxydiphenylsulfone, 2,4′-dihydroxydiphenylsulfone is dissolved as a dialkali metal salt and 4,4′-dihydroxydiphenylsulfone is separated as a monoalkali metal salt in the presence of a hydroxide of an alkali metal, is proposed as the process for efficiently producing highly pure 2,4′-dihydroxydiphenylsulfone. However, in accordance with this process, great amounts of sodium hydroxide and sulfuric acid are used for the separation, and the efficiency is small from the standpoint of the cost.
The present invention has an object of providing a process for efficiently and economically advantageously separating 2,4′-dihydroxydiphenylsulfone from a reaction mixture comprising 4,4′-dihydroxydiphenylsulfone and 2,4′-dihydroxydiphenylsulfone and effectively utilizing phenolsulfonic acid contained in the reaction mixture as the raw material.